The invention relates generally to a belt reel for safety belts in motor vehicles. More specifically, the invention relates to a belt reel which has a spindle onto which the safety belt is wound and which has a portion which is latched to a fixed frame and prevented from rotating in response to the triggering of an activation device. The reel contains a force limiting element which reduces the force which is applied to the seat belt following the latching operation.
Belt reels that prevent winding out of a safety belt in the event of an accident by a latching a spindle to fixed frame or structure, have been previously proposed.
These belt reels are provided with at least one force-limiting element, which limits the force acting on the person and the safety belt respectively to a given value. The avoids injury to the occupant by the safety belt. The safety belt is wound out from the belt reel while overcoming a resistance produced by the force-limiting element. Energy is dissipated by the force-limiting element.
The force-limiting element in such a belt reel can have one or more components which become mechanically deformed when the pre-determined force is exceeded. Torsion bars are, for example, used as such deformable components and are arranged to twist in the direction of the belt winding out due to a rotational movement of the spindle.
It is an underlying object of the invention to provide a belt reel which provides increased safety over an increased range of impact speeds.
A belt reel according to the present invention includes force limiting element which takes the form of a hydraulically operating element. When rotation the belt reel is blocked, the wind out length of the safety belt is proportional, on the one hand, to the mass of the person to be secured and, on the other hand, to physical values resulting from the properties of the hydraulically controlled element.
An advantage of such an arrangement is such that the wind out length of the safety belt, in contrast to known force limiting elements, does not depend on the impact speed of the motor vehicle.
The energy at which a person to be secured moves in the event of a rear-end shunt amounts to E=mv2/2 (m: mass of the person; v: impact speed). This energy E results in a force F acting onto the safety belt which is wound out by a wind out length s.
With a force F held constant by a known rotation-resisting element, it follows from E=Fs that the wind out length s is proportional to the square of the impact speed: sxcx9cmv2/2. This means that the safety belt can continue to wind out at higher impact speeds.
This can be compared with known force-limiting arrangements which can only be arranged to produce suitable control in a certain impact speed range, and wherein the force limitation does not work ideally in the event of impact speeds above and/or below this range.
In contrast, with a hydraulically controlled force-limiting arrangement, the force F, which acts on a displacement body by a fluid flow, is proportional to the square of the flow speed vfl(F=cApvfl2/2 where c: drag coefficient of a displacement body flowing through a fluid; A: cross-section area of the displacement body and p; density of the fluid) so that the energy required for the displacement of the fluid is also proportional to the square of the flow speed. With E=Fs, it therefore follows from scApvfl2/2=mv2/2, under the assumption vflxcx9cv, that sxcx9cm/cAp. The wind out length s of the safety belt is thus not a function of the impact speed v in a hydraulic force limiter.
A pre-determined wind out length of the safety belt can therefore be set by the adjustment of the hydraulic parameters which is influenced only by the mass of the person to be secured. A belt wind out process, which takes place in a defined and optimum manner at all imaginable impact speeds, thus ensures a constantly good force limitation and thus increased safety.
As a result of the squared dependence of the force F on the flow speed Vfl(F=cApvfl2/2) in a hydraulically operating force-limiting element, due to vflxcx9cv the force which acts on the safety belt or on the person to be secured is also proportional to the respective speed after the start of the impact delay. The force curve is therefore not constant or even increasing as with known force limiter systems, but degressive (like the speed) and easily adjustable in its curve as in known hydraulic systems. The person to be secured is thereby exposed to a harmonious force developmentxe2x80x94as with a careful braking, when the force at the brake pedal is slowly reduced.
Advantageous embodiments of the invention are described in the dependent claims, the description and the drawings.
For instance, in a preferred embodiment of the invention, a hydraulic fluid can be displaced by a force in excess of a predetermined value which acts acting on the spindle. Due to this design, hydraulic fluid is used instead of mechanically deformable components for the force limitation, which simplifies the mechanical structure of the belt reel and can result in a substantial weight saving.
It proves advantageous here for the hydraulic fluid to be displaceable by a displacement element either within a fluid container or out of a fluid container. The hydraulic fluid can thus be located in an enclosed container in which it is either circulated by the displacement element or moved from one region of the container into another. Alternatively, it can be forced out of the container through an opening. These methods require, with a suitable selection of the hydraulic fluid, sufficient work to be done that a given amount of energy can be converted into work via the displacement of the hydraulic fluid.
An embodiment is particularly preferred in which the fluid container is divided into two regions and the hydraulic fluid can be displaced from one region into the other. Since the hydraulic fluid cannot leave the fluid container in such an arrangement, there is no need to provide an additional collection container for the displaced hydraulic fluid. A more compact constructional design of the belt reel can thus be realized which in turn, saves space and weight.
The fluid container is advantageously divided into two regions by the displacement element. The displacement element thus has two properties which differ from one another in the displacing and space-dividing effects. The take-over of two functions by one element also provides a weight saving and a simplification of the design.
In a further embodiment of the invention, the displacement of the hydraulic fluid takes place via a restrictor element. The force required for displacement of the hydraulic fluid can be increased by the restrictor element. The restrictor element additionally provides the possibility of precisely setting the force required to displace the hydraulic fluid by the exact design of the restrictor element. An additional parameter for the optimization of the force limitation is herewith provided in addition to the physical properties of the hydraulic fluid.
It is particularly advantageous if the displacement element is simultaneously achieved by the restrictor element. It also performs two tasks in this case. If the displacement element divides the fluid container into the two previously mentioned regions, then it even provides three functions: a restrictor, a space divider and a displacer, whereby a compact and simple design of the belt reel is achieved.
In a further embodiment of the invention, the force limiting element has a means which, after a first activation, returns the element to a state which is substantially similar to a state prior to the activation. The force limiting element is thereby substantially reversible and can, for example, be activated a plurality of times such as in the event of a double impact.
It is possible for the hydraulic fluid to have an electro-viscous liquid property. The viscosity and thus the energy absorption in the force limiting element can be set in such a liquid by means of an electric signal. An individual adjustment of the force limiter to the weight of the person to be secured is made possible in this way, for example by means of a seat occupation detection.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.